As an elastic element which is used in a balancer device or the like and can adjust its own generating force thus coping with a change in load weight, there have been known a metal spring (see, e.g., Patent Literature 1 and Non-Patent Literature 1) and an air spring (see, e.g., Patent Literature 2). There has been also proposed a method in which a generating force of an air spring which changes corresponding to a displacement is made uniform using a non-true-circular disk (see, e.g., Patent Literature 3).